Find the surface are of the pyramid

Which of the following is most likely the next step in the series? a3z, b6y, c9x, d12w, е15v, f18u

The given dot plot represents the average daily temperatures, in degrees fahrenheit, recorded in a town during the first 15 days of september. if the dot plot is converted to a box plot, the first quartile would be drawn at __ , and the third quartile would be drawn at __ link to graph: .

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?